1. Field of the Invention:
The present invention relates to a method of and an apparatus for providing a thermoplastic film with continuous or discrete slits extending in the longitudinal or transverse direction thereof.
2. Prior Art:
In the manufacture of reticular webs, a variety of methods and apparatus have been known for providing a thermoplastic film with continuous or discrete slits extending longitudinally or transversely between the opposite edges of the film before the film is stretched transversely or longitudinally into a reticular web.
Among such known apparatus, razor cut slitters and highspeed rotary cutters have been found to be disadvantageous in that their sharp cutting edges are liable to be worn out and become dull in a relatively short period of use when employed in slitting a non-stretched thermoplastic film, because the cutting edges undergo much abrasion by being forced along the film surface. With such worn-out or dull cutting edges, the film is locally stretched or sometimes cracked at portions adjacent to the slits formed, and hence is ruptured or torn apart at the defective slits as it is stretched into a reticular web. In a score cut slitter, circular cutting edges pressed against a hard peripheral surface of a roll are worn out at a rapid rate, moreover, the roll surface is liable to be damaged by the cutting edges. As shear cut slitters utilize a cooperating pair of bottom and top blades, a film is liable to be cracked at the ends of slits formed with the blades. Ordinary heat cut slitters have the drawback that slits formed with a hot blade are irregular in shape due to scratches made by cutting blades stained with fused film material sticking onto them, or are partly closed by rejoining.